This thesis deals with fundamental issues underlying the systematic construction of behaviour models of physical systems, especially man made engineering systems. These issues are important for the design and development of effective computer aided modelling systems providing high-level support for the difficult task of modelling. In particular, the thesis is about conceptual modelling of physical systems, i.e. modelling characterized by the explicit use of well defined abstract physical concepts. An extensive review of conceptual modelling is presented, providing good insight into modelling in its own and forming a useful reference for the development of computer aided modelling systems.An important contribution of this work is the extension of the conceptual modelling framework by an ideal switch concept. This novel concept enables a uniform and systematic treatment of physical systems involving continuous as well as discrete changes. In the discussion of the switch concept, the bond graph approach to modelling is used as a specific example of a conceptual modelling approach. In this discussion, the bond graph version of the switch is presented. This switch element fully complies with the standard bond graph modelling formalism.The thesis consists of six papers. The first paper contains an extensive review of (traditional) conceptual modelling. The next three papers deals with the ideal switch concept and the remaining two papers discuss an application. Four of these papers have been published in proceedings of international conferences.